free radical

an atom or molecule having at least one unpaired electron: free radicals are usually very reactive and unstable, and can damage healthy body cells

See free radical in American Heritage Dictionary 4

noun
An atom or group of atoms that has at least one unpaired electron and is therefore unstable and highly reactive. In animal tissues, free radicals can damage cells and are believed to accelerate the progression of cancer, cardiovascular disease, and age-related diseases.

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